Linux-kernel mailing list archive 2000-32,: Re: NTFS-like streams?

# Re: NTFS-like streams?

Michael Rothwell (rothwell@holly-springs.nc.us)
Fri, 11 Aug 2000 16:48:13 -0400

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Christopher Vickery wrote:

> I'm interested in implementing a system that associates
> meta-data with inodes, and would like to know if it has
> already been done or is in the works.

None has. There's two different ways of doing it currently;
the BeOS way and the NT way. As you said, NT makes a
namespace augmentation, using the ":" character to
deliniate attribute names from file names. This is called
"named streams". BeOS does not do that, but provides
special accessor functions instead; this is called
"extended attributes." They both accomplish the same
goal though: keeping extra data about a file with the file.

Urban Widmark wrote:

> Erm. What is the point?
> If you read x.bmp do you eventually start reading x.bmp:thumbnail?

The point is to be able to store and manage extra data about a file
with the file. On BeOS and NT, if you copy, move, rename, etc. a
file, the metadata (stream/attribute data) stays with it. The Mac
uses its streams to store file type information, icon, icon
position, comments and more. That's all done in a standard way and
is transparent to applications simply wanting to read the data fork.

I'm not sure what your question is asking. You don't have to read
x.bmp:thumbnail if you do not want to. But if you want to view the
thumbnail, it's there. For instance, on the Mac, you can have a
comment stored with each file as an extended attribute. Applications
can choose to use this data if they want, but htye do not have to. It
simply makes things nicer for the user.

Alexander Viro wrote:
> There is a sane and completely portable scheme - use directories. It's

> less brittle and will work with the standard tool on normal
filesystems.
> Why not use it?

Because it's more brittle. There is no reliable way to keep the extra
data
associated with a file actually with the file. Using directories at best

keeps it _near_ the file. Plus, the implementation of "extended
attributes"
using regular directories would probably be highly
application-dependant.

Alexander Viro wrote:
> The main problem is that rename(), unlink(), etc. are too
> fine-grained for such beasts. It's not that filesutils will barf on
names.
> Just suppose that you are saying mv * foo/bar. Shell will substitute
> the whole whorehouse - filename1:attribute1, filename1:attribute2,
etc.
> and in the best case mv will call rename() on each of them (just how
should
> we interpret "move the attribute A of file B into directory
foo/bar"?).
> In the worst it will try to _copy_ them and then remove the old ones.
> Moreover, you are going to confuse the hell of VFS and every program
out
> there (let alone POSIX, etc.) if rename() on one name will move a
bunch of
> other. It just does not happen.

The whole whorehouse wouldn't be moved, or shell-subsitutited. If you
had a
file, "myfile", which had three streams named "1", "2" and "3", on a
streams-
aware filesystem, all you see in "ls" is "myfile", not "myfile:1",
"myfile:2",
etc. And to copy or move the file, you just "copy myfile" and the
streams go
with it.

Linux can actually be equipped with both BeOS-type nad NT-like systems
for
accessing named streams/extended attributes. A filesystem can tell the
VFS
taht it supports named streams. Otherwise, named streams are not
supported.
This is similar to NTFS vs FAT on Windows NT. If you copy a file with
streams
from and NTFS volume to a FAT volume, the streams are thrown away. There
is no
attempt made to support streams on a filesystem that does not support
them.

Similarly, an enhanced Linux VFS can support or not support streams,
depending
on the capabilities of the underlying filesystem. The Macintosh actually
makes
and attempt to preserve extended attributes on non-HFS filesystems
through
"AppleDouble" and "AppleSingle" schemes. While that is interesting, I
prefer the
cleaner NT way of dealing with it -- don't kludge extra features onto
that don't support them.

Granted, there will still be issues with userspace tools like cp. Rm and
mv
will not have an issue, unless mv is moving across volumes. Cp and mv
will
have to enumerate and copy all streams. Rm will not need to enumerate
streams;
if you delete a file with streams, all the streams go with it.

On filesystems that support fast indexing -- JFS, XFS, Reiser -- streams
can be
used as in BeOS to provide indexing capabilities. BeOS email
applications, for
instance, use extended attributes to store information about a message
-- the
subject, sender, etc. This allows any application to look up email
messages by
those criteria simply by reading the attributes, rather than parsing
mail
headers.

-Michael

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- Next message: Theodore Y. Ts'o: "Re: Linux 2.4 Status"
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- Maybe in reply to: Christopher Vickery: "NTFS-like streams?"
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